1. Field of the Art
The present invention relates to a vibration damper installed on a subject body for reducing vibration of the subject body. More particularly, the present invention is concerned with such a vibration damper having oscillating force generating means therein.
2. Discussion of the Related Art
As means for damping or reducing vibration of a subject body such as a vehicle body which is subject to vibration, a dynamic damper is widely known. In recent years, for obtaining a further improved vibration damping or reducing effect, JP-A-61-220925 and JP-A-3-292219 propose an active-type vibration damper which is adapted to reduce or control the vibration of the subject body by applying, to the subject body, an oscillating force generated by an actuator.
In the conventional dynamic damper, it is extremely difficult to obtain a desired damping effect when the frequency of the vibration to be damped changes depending upon various factors. For example, a body of an automotive vehicle is subject to various kinds of vibrations depending upon changes in the engine rotating speed and running speed of the vehicle. Further, the dynamic damper which functions as a secondary vibration system is adapted to reduce the vibration of the subject body as a primary vibration system, by absorbing the vibration energy of the subject body. Thus, the conventional dynamic damper is not satisfactory for assuring a high vibration damping effect.
In the active-type vibration damping device, it is required to employ an actuator capable of generating an oscillating force which is large enough to assure a high damping effect. However, where the subject body has a large size and a high rigidity as in the case of a vehicle body, the energy of the vibration of the subject body is accordingly large. In this case, the actuator tends to be large-sized, and the electric power consumed by the actuator is inevitably increased. In addition, it is difficult to apply a sufficient oscillating force to the large and highly rigid subject body. Thus, the conventional active-type vibration damping device is not always satisfactory in its damping effect.